For decades, mine operators in Northern Canada have stored waste rock and tailings waste—the “pulverized rock slurry” byproduct of mineral processing that’s filled with skeevy chemicals like arsenic, lead, and mercury—in frozen dams reinforced with permafrost, an option far cheaper than constructing artificial structures to house the goop. But if such walls thaw, allowing air and water to interact with the highly reactive tailings, widespread “acid mine drainage” (AMD) could occur. Such a process can generate sulphuric acid and result in the leaching of heavy metals into nearby soil and water sources.